Since the synchronization link, wavelength calibration link and other 8 running fiber links of classical communication in the same bundle with the QKD link. The signals running in these links would leak into the QKD link as noise. We measured the spectral distribution of the crosstalk from the 8 running fiber links of classical communication, which is shown in Fig. REF , the crosstalk is mainly distributed between 1548.5 nm and 1549.5 nm. Therefore we set a wavelength of 1550.12460 nm of the QKD sources to keep away from the main distribution range of crosstalk from the 8 running fiber links of classical communication, and insert a 100 GHz DWDM (1550.12 nm) in measurement station to filter the noise. Meanwhile the leakage of the synchronization pulses is also filtered out with the DWDM. In addition, another important source of noise is the leaked wavelength calibration light. As the wavelength of the calibration light is the same as the signal, we cannot filter them with the DWDM. To reduce this noise, we adopt the time-multiplexing method. We first calibrate the wavelength difference of the two independent light sources before the experiment, with strong CW reference light in the fiber; Then when the experiment is running, the strong reference is blocked, and the wavelength between the two light sources are left to free drifting. With stable light sources, the wavelength difference will be small for the experiment between the wavelength calibration cycles. We usually calibrate it about every one or two hours. Finally, as is shown in Tab. REF , we tested all the noises above and the dark count of the SNSPDs, and estimate the noises from re-Railey scattering noise [1]}. Sum all the above noise sources together, we achieve about 24 cps (count per second) noise counts for each detector. We further use a 350 ps narrow gate, to reduce the noise probability per gate, achieving a dark count rate of \(7 \times 10^{-9}\) .
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